Tsuruga faults inactive, says study

10 June 2014

Geologic faults that run beneath the Tsuruga plant are not active, said an international expert team which also recommended better communication between plant owner Japan Atomic Power Company (Japco) and regulators.

The Nuclear Regulation Authority (NRA) is enacting much more stringent rules on nuclear safety than the previous regime. One is a requirement for nuclear operators to satisfy the NRA that known geologic faults near power plants have not been active for some 120,000-130,000 years.

Proving this negative to NRA's satisfaction has been a challenge, and for the utility Japco this has meant making enormous excavations alongside its Tsuruga power plant to examine the status of two faults that run nearby.

Presenting its second set of findings this week, an international expert team concluded that there was no evidence that the faults near Tsuruga were active. They pointed to several layers of sediment and one of volcanic ash that were laid down on top of the faults and have rested undisturbed since their last movement, more than 127,000 years ago.

The team noted that its latest conclusions were the same as its first, which were presented to the NRA in August last year. Additional investigation demanded by NRA had only yielded "even more clear evidence" on top of last year's "pervasive and powerful scientific evidence" that indicates the faults are not active.

The Tsuruga study was independently performed for Japco by the 'International Review Group' and the 'Third Party Review Group', consisting of Kelvin Berryman, Woody Epstein, Nicola Litchfield, Koji Okumura, Neil Chapman and Hirokazu Kato.

Problems in communication between Japco and the NRA were apparent: The team noted that NRA "has been silent" on one of the faults since last June and that "there is clearly need for better dialogue... there is a sound scientific basis for Japco and the NRA to enter a dialogue on continuing and improving the seismic safety evaluation and management of Tsuruga."